The research proposed in this application will be primarily performed in Santiago, Chile at the Universidad de Chile in collaboration with Dr. Jimena Sierralta as an extension of NIH grant RO1 NS42629.The long-term goal of the PI's research is to understand the molecular mechanisms by which synapses are assembled using a genetic approach in the glutamatergic neuromuscular junction of Drosophila. An important finding emerging from this investigation is that a PDZ-containing scaffolding protein of the PSD-95 family, DLG, is essentially required to properly localize a number of synaptic proteins. The central aim of the parent grant is to characterize the role of two DLG interacting proteins, Scribble and GUK-holder, in the process of synapse assembly. Previous studies of the dig locus identified the presence of a single transcript, dig-A, which is present both in synapses and epithelial cells. However, recent studies by Dr. Sierralta demonstrate extensive alternative processing of transcripts originated from the dig locus. Very interestingly, a group of splice variants are excluded from epithelial tissue and are specifically expressed in the nervous system. Moreover, a subset of isoforms contains an N-terminal extension similar to mammalian SAP97, which regulates SAP97 localization as well as intramolecular interactions that control the binding between SAP97 and its partners. The main goal of the proposed collaborative research is to investigate the role of these novel splice variants in different aspects of synapse formation and maturation. The work involves the study of the expression of DLG isoforms containing the $97N domain during the development of synapses, the selective disruption of dig gene products containing the $97N domain during synapse development, and the study of the role of this domain in Dig binding to its partners. We envision that the novel dimension provided by the observations in the foreign lab will dramatically enhance our understanding of DLG during synapse formation, and therefore will increase the knowledge obtained during the execution of the parent grant. [unreadable]